Compact emissivity and temperature measuring infrared detector

ABSTRACT

A hand-held, fleet deployable infrared camera with integrated hardware and software providing real time processing of infrared images. The camera senses variable temperature images over a selected object of interest and senses variable emissivities over the object. The camera also measures and corrects for reflected environmental radiation as well as corrections for the atmospheric path through which the object is viewed. A calibrated reference patch having known emissivity and reflectance is attached to an object of interest and viewed through the camera. The calibrated patch is used to provide correction for the environmental radiation reflected off the object. Once the environmental radiation correction is known, it can be used to correct measurements taken from the rest of the object of interest.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a compact, man-portable, infrared camera thatis usable in an operational military environment by military personnel.The camera uses two-color radiometric techniques to determine theemissivity and temperature images displaying the spatial distribution oftemperature and emissivity for extended objects. The process includescorrections for the atmospheric path through which the object is beingviewed and for the environmental radiance being reflected off theobject. The camera is designed for the decoupling of the reflected andemitted radiation from the object and the direct solution for theemissivity.

2. Background of the Invention

All surfaces emit thermal radiation. However, at any given temperatureand wavelength, there is a maximum amount of radiation that any surfacecan emit. If a surface emits the maximum amount, it is known as ablackbody. A blackbody has an emissivity of 1.0 at all temperatures andwavelengths. Most surfaces are not blackbody emitters, and emit somefraction of the amount of thermal radiation that a blackbody would.Emissivity is the ratio of radiation emitted by a surface and thetheoretical radiation predicted by Planck's law.

The problem to be solved is to develop a robust, hand-held infraredmeasurement device to evaluate the infrared characteristics of an objectof interest, such as aircraft and other vehicles. There are currentlyavailable laboratory instruments capable of measuring emissivity. Theseinstruments are large, difficult to operate, and require careful controlof the laboratory environment. As such, they are unsuitable for robust,simple, and man-portable field operations. There are also satelliteinstruments that attempt to measure emissivity from orbital platforms.These devices have been deployed for many years. They typically usebroadband radiometers and/or a collection of narrow band measurements.Often they also include some sort of ground truth measurement to supportthe space-based measurement. These instruments are again unsuitable forrobust man-portable operations.

Field instruments do exist, however. One class of instruments normallyattempts to measure emissivity by measuring reflectance and calculatingthe emissivity based on this measurement. Surface emissivity is measuredindirectly by assuming that ε=1—reflectivity. In general, a singleenergy bounce is measured and the reflected energy is measured.Typically, a large intensity laser is used as a radiation source and thestrength of the reflected intensity is measured. This allows for thecalculation of measured reflectance and hence resultant emissivity atthe wavelength of the laser. These laser instruments can work adequatelyin radiation bands in which there is no emission, such as the visibleband, but become problematic if thermal emission sources in the objectbeing measured must compete with reflected laser intensities. Theselaser devices also suffer from the fact that they are inherently basedon measurements at a single wavelength or, at best, a small number orwavelengths and they generally do not provide large field of viewemissivity images of the object of interest.

Another class of field instruments for measuring emissivity containssingle band radiometers. These unfortunately require knowledge of thesurface temperature and again suffer from an inability to unravelreflected and emitted light from the source.

For example, U.S. Pat. No. 5,272,340 to Anbar shows an infrared imagingsystem for simultaneous generation of temperature, emissivity, andfluorescence images that determines temperature, reflectivity, andfluorescence of a surface. U.S. Pat. No. 5,868,496 to Spitzberg shows amethod to calculate surface temperature from an object by measuringradiated energy in multiple wavelength bands. U.S. Pat. No. 4,659,234 toBrouwer et al. shows a method to correct emissivity readings for aradiation thermometer by measuring radiated energy at two wavelengths.U.S. Pat. No. 4,974,182 to Tank shows a method for measuring theemissivity and temperature of an object by successive determination ofradiance in multiple wavelength bands.

Thus, although substantial effort has been devoted in the art heretoforetowards development of methods to measure temperature and emissivity,there remains an unmet need for a robust device which is easier to useand which can be deployed to an operational military environment.Likewise, there remains an unmet need for a method to measuretemperature and emissivity that corrects for atmospheric conditions andenvironmental radiance.

SUMMARY OF THE INVENTION

The present invention provides a hand-held, fleet deployable infrareddevice having a camera with integrated hardware and software providingreal time processing of infrared images. The device measures andcorrects for reflected environmental radiation from a selected object ofinterest and corrects for the atmospheric path through which the objectis viewed. The device senses and displays variable temperature imagesover the object of interest. It also senses variable emissivities overthe object. The output from the device is an image of the object ofinterest with representations of temperature and emissivity over theentire object of interest.

A calibrated reference patch having known emissivity and reflectance isattached to an object of interest and viewed through an infrared camera.The calibrated patch is used to provide correction for the environmentalradiation reflected off the object. Once the environmental radiationcorrection is known, it can be used to correct additional measurementstaken from the rest of the object of interest.

It is, therefore, an object of the present invention to enable anemissivity and temperature measuring infrared device that avoids thedisadvantages of the prior art.

It is another object of the present invention to enable obtainingtwo-color radiometric image measurements. It is a related object of thepresent invention to define a system for removing reflected radiation,correcting for atmospheric path absorption, and calculating temperatureand emissivity spatial distributions from two-color infrared imageradiometric measurements.

It is another object of the instant invention to enable an emissivitydetector that incorporates real-time image processing hardware andsoftware. It is a related object of the instant invention to enable adevice integrating an infrared camera, digital image processinghardware, range measurement, and a user interface into an easy-to-use,robust, hand-held system.

In accordance with the above objects, a robust, emissivity andtemperature measuring device is disclosed. Some of the advantages of thedevice include a camera that can sense variable temperature images overan object, a camera that can sense variable emissivities over an object,and a system that can sense and correct for reflected environmentalradiation.

The various features of novelty that characterize the invention will bepointed out with particularity in the claims of this application.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features, aspects, and advantages of the presentinvention are considered in more detail, in relation to the followingdescription of embodiments thereof shown in the accompanying drawings,in which:

FIG. 1 shows an isometric view of a detector in accordance with oneembodiment of the present invention;

FIG. 2 shows features of the two-color measurement concept;

FIG. 3 shows features of the measurement concept using a calibratedpatch;

FIG. 4 illustrates the calculations to determine environmental radiancein accordance with one embodiment of the present invention;

FIG. 5 illustrates the calculations to determine surface temperature andemissivity in accordance with one embodiment of the present invention;

FIG. 6 is a flowchart of the software functions in accordance with oneembodiment of the present invention;

FIG. 7 is a functional block diagram of the system according to oneembodiment of the present invention;

FIG. 8 shows a filter wheel interface according to one embodiment of thepresent invention;

FIG. 9 illustrates features of the filter wheel of FIG. 8; and

FIG. 10 shows the timing sequence for a filter wheel of FIG. 8 accordingto one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention summarized above and defined by the enumerated claims maybe better understood by referring to the following description, whichshould be read in conjunction with the accompanying drawings in whichlike reference numbers are used for like parts. This description of anembodiment, set out below to enable one to build and use animplementation of the invention, is not intended to limit the enumeratedclaims, but to serve as a particular example thereof. Those skilled inthe art should appreciate that they may readily use the conception andspecific embodiments disclosed as a basis for modifying or designingother methods and systems for carrying out the same purposes of thepresent invention. Those skilled in the art should also realize thatsuch equivalent assemblies do not depart from the spirit and scope ofthe invention in its broadest form.

Referring to the drawings, FIG. 1 shows a depiction of a detector,indicated generally as 7, according to the present invention. Thedetector 7 is compact, man-portable, and usable in an operationalmilitary environment by military personnel. Detector 7 has a case 10that includes an infrared camera portion 13 having an optics portion 16.Detector 7 also includes a user interface portion 19 and a displayportion 22. In some embodiments, detector 7 may also include handles 25,26 and a camera optics guard, such as 29. In a preferred embodiment,processing components for the system as taught herein are enclosedinside case 10. The algorithms taught herein may be installed on circuitboards within case 10.

Detector 7 uses two-color radiometric techniques to determine theemissivity and temperature images displaying the spatial distribution oftemperature and emissivity for extended objects. The process ofdetermining temperature and emissivity includes corrections for theatmospheric path through which the object is viewed and for theenvironmental radiance reflected off the object. In fact, the detector 7is designed to allow for decoupling of the reflected and emittedradiation from the object and the direct solution for the emissivity.Detector 7 presents the user with a large field-of-view of bothtemperature and emissivity images on display portion 22. These imagescontain tens of thousand pixels each representing average equivalentblackbody temperatures and emissivities over some region of the objectof interest.

The infrared camera portion 13 collects photons with its optical systemand focuses those on a focal plane array sensor. The sensor elementsconvert the photons to electrons that are then converted throughinternal processing mechanisms into digital counts. The output of thecamera 13 is then digital counts as a function of the scene to which theoptics portion 16 is directed. Before the digital counts can be useful,however, two steps are required: uniformity correction of the focalplane array and radiometric calibration.

The focal plane array (sensor) is made up of many individual pixels thateach collect radiation from a particular location in the scene. Thesepixels inherently do not respond the same. Hence, a uniform input intothe optics portion 16 would present a grainy output on the displayportion 22. This output is corrected by a software algorithm so that auniform input results in a uniform output. This conversion is called auniformity correction.

After the uniformity correction, the output from the camera portion 13is uniform. However, this output is in digital counts. Digital countsare not useful for analysis because the relationship between the digitalcounts out and the photons into the optical portion 16 is unknown. Theprocess of radiometric calibration relates the digital counts out to auseful measure of the photons in.

In the system taught herein, the non-uniformity correction and theradiometric calibration are performed in one step using a blackbody thatemits radiance as a function of its temperature according to Planck'sfunction. The temperature of the blackbody is determined and theblackbody is placed directly in front of the camera optics 16 to allow aknown radiance to be collected by the camera 13. The digital countsoutput associated with such known radiance are recorded for a firsttemperature. The temperature of the blackbody is changed and the processis repeated for another known radiance. The digital counts associatedwith the second radiance are recorded. Based on these two associationsof digital counts and radiance, a linear relationship between radianceand digital counts is computed, which is the calibration equation. Aseparate equation is calculated for each pixel in the focal plane array.It should be clear that the process of doing radiometric calibration inthis fashion will provide not just a calibrated output but also auniform output.

As explained above, the radiometric calibration calculated for eachpixel is a linear relationship between radiance into the lens anddigital counts out of the camera electronics. The linear relationshipbetween digital counts and radiance is uniquely defined by its slope (orgain) and y-axis intercept (or offset). Once calibrated, the camera hasa very stable gain. That is, the slope of the calibration line does notchange significantly with time. Unfortunately, the intercept (or offset)does drift. To correct for the drift of the offset, a calibration lenscap is placed over the lens and the radiance is recorded. The lens caphas a known temperature and presents a uniform radiance through theoptics to the focal plane array. This uniform radiance can then be usedto correct the offset for drift.

The camera described herein is capable of measuring light withwavelengths from 3-5 microns. The two non-overlapping infrared bandsused in the present invention are selected to optimize the choice withinthe capability of the camera. The infrared bands were selected to notuse wavelengths of light in which the atmosphere is not transparent andto reduce the sensitivity to noise. Since there is a carbon dioxideabsorption band from about 4.15 to 4.35 microns, that range is avoided.Furthermore, studies showed there is less sensitivity to noise in the4.5 to 5 microns band. In a preferred embodiment, the bands arenominally 3-4.15 microns and 4.5-5 microns.

FIGS. 2 and 3 show a depiction of the basic measurement concept oftaking two calibrated infrared images of an object in selected infraredbands. While FIGS. 2 and 3 show the infrared sensor as a separateelement from the processor, in a preferred embodiment, both elementswould be enclosed in a single case 10. The images are corrected foratmospheric transmission, and then calculations are performed on theimages to generate temperature and emissivity images. An integral partof the measurement concept according to the present invention is thecorrection for environmental radiation reflected off the target. Thiscorrection is done using a calibrated patch and a thermal probe. FIG. 3shows the same object as FIG. 2 with a calibrated patch located on thenose of an aircraft in preparation for a radiometric measurement of theaircraft's nose section by the two-color infrared detector system asdescribed herein. The calibrated patch attached to the surface of theaircraft will be in the field of view of each image taken and processedby the system. The calibrated patch has known emissivity andreflectance. The temperature of the patch is measured with a thermalprobe at the time of the measurement.

The atmospheric transmission correction is performed based on a standardatmospheric model (MODTRAN). A user selects the type of atmosphericmodel to be used from a menu, such as tropical summer or artic winter,depending on the location. The user also inputs the range from thedetector to the object of interest. Range (R) can be determined bymeasuring using a tape measure or other appropriate range finder. Thecorrection for atmospheric transmission is applied separately in eachband as shown by the equations in FIG. 4.

The correction for the environmental radiation reflected off the objectin each band relies on the calibrated patch. Since the emissivity andreflectivity of the calibrated patch is known and the temperature of thecalibrated patch can be determined, the environmental radiance in thevicinity of the calibrated patch can be calculated. Once the incidentenvironmental radiance is determined based on measurements of thecalibrated patch, this environmental radiation is then used to correctthe measurements in the rest of the scene.

FIG. 5 illustrates the equations for measuring temperature andemissivity using two-color thermometry by decoupling reflected andemitted radiation. As described above, the environmental radianceincident on the object is measured using the calibrated patch. Thesoftware algorithms then are able to manipulate the measurementequations from the two bands to solve for the objects temperature. Basedon the temperature and the incident environmental radiance theemissivity of the object can then be computed.

FIG. 6 is a flow diagram of the algorithms used to calculate thetemperature and emissivity images. The Algorithm Flow Chart gives anoverview of the software used in calculating the temperature andemissivity of the unknown object. The process starts with a number ofinputs and pre-calculated functions. The inputs are:

-   -   Range: distance from the camera to the object being measured    -   Atmospheric model selection: Pick a standard MODTRAN model        (Arctic summer for example)    -   T_(patch): Calibrated patch temperature as measured by a thermal        probe    -   Δλ₁ and Δλ₂: Wavebands of the two camera spectral filters    -   Patch size (y×z): Number of pixels being averaged for the        measurement of the background environment    -   T_(lenscap): Measured lens cap temperature    -   ε_(lenscap): Pre-measured and calibrated lens cap emissivity    -   m₁ and m₂: Arrays of radiometric gains in the lower and upper        bands as determined during the laboratory calibration    -   ε₁ and ε₂: Known emissivities of calibrated patch in the two        wavebands    -   ρ₁ and ρ₂: Known reflectivities of the calibrated patch        The definitions of the quantities in the calculations are:    -   T=f(ratio): Temperature versus ratio lookup table    -   τ₁ and τ₂: Average atmospheric transmission from the MODTRAN        atmospheric model    -   J_(1p) and J_(2p): Emitted radiance from the calibrated patches        as calculated from Planck's function    -   DIMD₁ and DIMD₂ (x₁ and x₂): Two arrays of digital counts output        by the camera's focal plane array in two successive frames (one        for each band) during the actual object measurement    -   DIMD₁ and DIMD₂ ({circumflex over (x)}₁ and {circumflex over        (x)}₂): Two arrays of digital counts output by the camera's        focal plane array in two successive frames (one for each band)        during the lens cap measurement.    -   b₁ and b₂: Radiance calibration offset correction calculated        from the lens cap measurement    -   E₁/Ω and E₂/Ω: Arrays of radiance in each band as calculated        from the calibration and the measurement    -   S₁(y×z) and S₂(y×z): Arrays of calculated environmental radiance        in the two bands    -   S_(1av) and S_(2av): Average environmental radiance in the two        bands    -   J₁ and J₂: Calculated radiance (from Planck's function) in the        two bands    -   T: Temperature in Kelvin    -   Δλ₁ and Δλ₂: Wavebands 1 and 2    -   ratio_(m×n): Array of ratios measured for each pixel in each        band    -   T_(m×n): Array of temperatures calculated based on the        measurement    -   ε₁ and ε₂ (m×n): Array of emissivities output as a result of the        measurement and calculations (one array for each band)

The software receives the external inputs as described above as eitherimbedded data or as data that is keyed in by a user. Based on theseinputs, corrections for the atmospheric transmission are tabulated andmade ready for use in the environmental radiance and ratio calculations.In addition, calibration tables for each pixel in each band areprepared. A measurement starts with a lens cap measurement to prepare anarray of offset corrections for use in the calibration. Once this iscomplete, the calibration tables are ready for use. The calibrated patchis then positioned on the object of interest and the camera measures aregion of the object that includes the calibrated patch. The calibratedpatch measurements are input to the environmental radiance calculation.All measurements are also sent into the ratio measurement and emissivitycalculation.

The first step is to use the known properties of the calibrated patchand the measured measurements to calculate the average backgroundradiance. This is then sent into the ratio table calculation for settingup the ratio table, the ratio measurement calculation, and theemissivity calculation function. The measured ratio is then matched witha value in the ratio lookup table to obtain the measured temperature ofeach point in the array. This temperature measurement is then sent tocalculate the emissivity. Finally, the emissivities in each band arecomputed for the entire array. Two color radiometric techniques have theadvantage of being able to measure temperature of a greybody objectoptically without prior knowledge of the object's emissivity. This is avery powerful approach to optical thermometry.

As shown above, the system forms ratios of the measured radiances ineach band (corrected for environment reflections). These ratios ofcorrected radiances are independent of unknown object emissivity andenable determination of the temperature of the object of interest fromcalculated lookup tables. Once the temperature is calculated, it can beused with the other measurements to calculate the surface emissivity.

FIG. 7 shows the functional block diagram for the system. In oneembodiment, the camera is a 3-5 micron infrared imager with a 256×320pixel focal plane array (FPA). The camera collects images at a 30 Hzrate. The optical system uses a lens with a rotating spectral filter 35(FIG. 8). Half of this circular filter passes radiation in the 3.0-4.15micron band and the other half passes in the 4.5-5 micron band. Thefilter must be rotated at a rate such that alternate frames collected bythe camera 13 are in different bands. That is, out of 30 frames, 15 arein one band and 15 in another and every other frame is in a differentband. A motor and controller 38 is used to rotate the filter at thecorrect speed and insure it is properly positioned. This controller 38is driven by a video synchronization signal from the camera. The outputof the camera goes into the processor 41 to do the computationsdescribed in the algorithm flow diagram. The processor 41 receivesinputs from the user interface portion 19 and outputs to the displayportion 22. In some embodiments, the display portion 22 may be an LCDvideo display. Additionally, in some embodiments, the video display maybe saved to an external permanent storage medium. The system is poweredby batteries and the electronics cooled by a fan.

FIG. 9 shows the camera and integrated optics and filter wheel. Thecustom rotating filter wheel is designed to alternate images between the3-4.15 (lower) band and the 4.5-5 micron (upper band). The filter wheeltransmits only in the lower band on one side and only in the higher bandon the other. Its rotation rate and position is synced with the framerate of the camera. Thus, a camera collecting images at 30 frames persecond as is standard would collect 15 images in our lower band and 15images in our upper band per second. These images are of coursealternated. In order to monitor the position and rate of rotation of thefilter wheel, a number of markings are placed on the wheel. A sensor isused to locate these marks and a controller operates the motor in afashion such that the images alternate between the two bands.

FIG. 10 illustrates the filter wheel 35 and camera synchronizationsequence. On startup, the filter wheel controller 38 initially drivesthe filter wheel 25 at approximately 900 RPM. The camera 13 sendsdigital images to the processor 41 at 60 Hz. The camera 13 also sends asynchronous 30 Hz signal to the filter wheel controller 38. The sensorin the camera optics 16 sends an identification pulse to the controller38 based on the position of the filter wheel 35. The filter wheelcontroller 38 adjusts the rotational speed of the filter wheel motor tocorrectly align the phase angle of the filter wheel 35 relative to thecamera frames. The filter wheel controller also sends filter wheelposition data to the processor 41.

Besides the main purpose of the present invention, which is to sense thetemperature of an image and then compute the emissivity distribution, itis also possible for this instrument to be used purely for calculatingthe temperature of an object. In cases where the emissivity is unknownbut locally constant or in which it varies over the scene, this will bea powerful tool. That is, standard thermal optical radiometry measurestemperature in cases where the emissivity is known. However, the presentinvention enables the capability to measure the temperature optically incases where the emissivity is not known and varies over the scene. In analternate embodiment, the integrated optics and filter wheel can bereplaced with an inherent two-color infrared sensor. In a furtheralternate embodiment, an infrared sensor with two (or more) separateoptical trains may be used. Each optical train would focus energy fromthe image in each band on one-half the focal plane array. Anotheralternate embodiment uses a hyperspectral imager. In some bands (such asthe visible), it is reasonable to use two independent sensors each withan appropriate spectral filter rather then one sensor with a rotatingfilter.

It is also important to emphasize that the spectral band to be used inany implementation of the system is a variable that is optimizeddepending on the temperature and emissivities to be measured and themeasurement environment. Examples include the possibility of droppinginto the visible or near infrared band to measure very hot objects orpushing out into the long wave infrared band to measure cooler objects.Again, the process of optimizing the filters and measurement device is anovel element in this technology.

The invention has been described with references to a preferredembodiment. While specific values, relationships, materials and stepshave been set forth for purposes of describing concepts of theinvention, it will be appreciated by persons skilled in the art thatnumerous variations and/or modifications may be made to the invention asshown in the specific embodiments without departing from the spirit orscope of the basic concepts and operating principles of the invention asbroadly described. It should be recognized that, in the light of theabove teachings, those skilled in the art can modify those specificswithout departing from the invention taught herein. Having now fully setforth the preferred embodiments and certain modifications of the conceptunderlying the present invention, various other embodiments as well ascertain variations and modifications of the embodiments herein shown anddescribed will obviously occur to those skilled in the art upon becomingfamiliar with such underlying concept. It is intended to include allsuch modifications, alternatives and other embodiments insofar as theycome within the scope of the appended claims or equivalents thereof. Itshould be understood, therefore, that the invention may be practicedotherwise than as specifically set forth herein. Consequently, thepresent embodiments are to be considered in all respects as illustrativeand not restrictive.

1. An emissivity detector comprising: a. a housing; b. a camera arrangedin said housing, said camera comprising: (1) a lens; (2) a plurality ofinfrared sensitive detector elements arranged as a two-dimensionalarray; and (3) means for alternating the band of wavelength of infraredsignals incident upon said detector elements; c. a processing circuit,including a memory adapted to have stored therein a plurality ofatmospheric models for selection, said atmospheric models being used tocorrect the atmospheric transmission path of infrared signals from anobject of interest to said camera; and d. a screen that is arranged todisplay an image representing emissivity of an extended object ofinterest.
 2. The emissivity detector of claim 1, wherein said band ofwavelengths alternates between 3-4.15 microns and 4.5-5 microns.
 3. Theemissivity detector of claim 2, further comprising: a filter wheelwherein one-half of said filter wheel passes radiation in the 3-4.15micron band and one-half of said filter wheel passes radiation in the4.5-5 micron band.
 4. The emissivity detector of claim 3, whereincorrection for the atmospheric transmission path is performed in eachwavelength band.
 5. The emissivity detector of claim 3, furthercomprising: means for rotating said filter wheel in front of said lensin synchronization with the frame speed of said camera.
 6. Theemissivity detector of claim 1, said processing circuit furthercomprising: software including equations to correct for environmentalradiation reflected off an object of interest and to calculate thetemperature profile of said object of interest.
 7. The emissivitydetector of claim 6, wherein said processing circuit further computesthe emissivity of the object of interest based on the temperature andthe environmental radiance.
 8. The emissivity detector of claim 1,further comprising: a user interface wherein a user can input data tosaid processing circuit.
 9. The emissivity detector of claim 8, whereinsaid user interface enables input of a selected atmospheric model. 10.The emissivity detector of claim 8, wherein said user interface enablesinput of the range between said detector and an object of interest. 11.A method for measuring emissivity and temperature of an object,comprising: a. providing a detector that receives energy in a pluralityof wavelengths; b. providing a patch, attached to said object whereinsaid patch has a known emissivity; c. determining the temperature of thepatch; d. measuring the range between the patch and said detector; e.correcting the received energy for the atmospheric path along the rangefrom the patch to the detector; f. decoupling reflected radiation fromthe radiation emitted by the object; and g. calculating the temperatureof the object.
 12. The method of claim 11, further comprising:calculating the emissivity of the object based on said calculatedtemperature.
 13. The method of claim 11 wherein said detector receivesenergy in two alternating infrared wavelength bands.
 14. The method ofclaim 13 wherein said two infrared wavelength bands are 3-4.15 micronsand 4.5-5 microns.
 15. The method of claim 11 wherein the step ofcorrecting the received energy of the atmospheric path includesselecting an atmospheric model based on the location of the object.